Field of the Invention
The invention relates to a method by which a measurement range of a laser scanner which is installed alongside a roadway and which is fixedly connected to a camera can be aligned with respect to the roadway surface.
Description of the Background Art
With the use of non-contact measuring devices for traffic measurements it is important for the profile of the measurement radiation and thus the position of the measurement range relative to the position of the roadway to be defined in a predetermined manner.
This applies to laser scanners, in particular, in which the measurement range is determined by a scanning angle range within which a laser beam is deflected, relative to a centre axis bisecting the scanning angle range, about an axis orthogonal to the scanning plane at the origin of the scanning angle range. Disregarding the divergence of the laser beam, the measurement range thus constitutes a delimited plane and its spatial position is determined by the arrangement and alignment of the laser scanner with respect to the roadway relative to features of the roadway.
Assuming that only reflection signals from vehicles are obtained if the scanning plane lies in a height range of approximately 45 cm to approximately 70 cm above the roadway surface, such that the laser beam impinges on the vehicle at a height of approximately 45 cm to approximately 70 cm, that the measurement range is generally intended to cover a plurality of lanes of a roadway and a vehicle travelling through the measurement range is intended to be detected with a minimum frequency, the laser scanner, which is installed at a different installation height with respect to the roadway margin in a situation-related manner as a result of different ground heights outside the roadway, has to be aligned optimally with respect to the roadway margin and with respect to the roadway surface.
In the present description, a laser scanner can be understood to mean a complete serviceable device having a frame for temporary or permanent installation, such as is used in traffic measurement. By means of a scanner mirror that rotates or pivots about a scanning axis, a pulsed laser beam coming from a laser source is deflected, within a predefined scanning angle range, about a centre axis bisecting the scanning angle range in a scanning plane. By means of the pulse time-of-flight measurement, from the reflection signals that arise as a result of reflection of the pulsed laser beam at areas (reflection points) of vehicles travelling through the measurement range, the distance of the reflection point is ascertained. On account of the reflective areas of the vehicle, during a scan the reflection signals from a vehicle describe a pair of path sections approximately perpendicular to one another, that is to say a right angle. This right angle shifts from scan to scan as a vehicle passes through, and the speed of the vehicle to be measured is determined from the shift of the right angles.
In order to achieve a sufficient accuracy for the speed, a sufficient number of shifts of the right angles are required. For this purpose, the vehicle has to cover the distance of a minimum path section in the measurement range in order to obtain a sufficient number of scans with reflection signals per vehicle. Therefore, as already explained, the measurement range, which is formed only by a delimited plane, has to be aligned optimally with respect to the roadway surface and with respect to the roadway margin. Furthermore, the alignment of the laser scanner has to be known in order to be able to draw unambiguous conclusions about the position of the measured vehicle from the reflection signals. That is to say that the position of the pulsed laser beam has to be able to be assigned to a known vertical angle with the roadway surface and a known horizontal angle with the roadway margin at each measurement instant, in order to be able to deduce a position of the reflection point relative to the roadway from a distance value assigned to the measurement instant.
The alignment of a laser scanner with respect to a roadway can be described, in principle, by three parameters, the rolling angle, the pitching angle and the azimuth angle (yaw angle). In accordance with the prior art, the laser scanner is installed at an installation height which lies within the height range in which vehicles can be reliably detected. According to the invention, a laser scanner is also intended to be able to be installed at a greater installation height. For this purpose, a pitching angle is set depending on the installation height. The rolling angle is intended always to be equal to zero.
The rolling angle can describe the inclination of the scanning plane relative to the roadway surface, which is understood as a plane. The rolling angle is equal to zero or is eliminated if a laser scanner longitudinal axis of a laser scanner coordinate system defined by the laser scanner lies in a plane parallel to the roadway surface. The rolling angle can also be explained as the angle formed between the centre axis of the laser scanner and a perpendicular—positioned into the scanning plane—to a straight line of intersection formed between the roadway surface and the scanning plane. That is to say that the rolling angle is equal to zero or eliminated if said straight line of intersection is orthogonal to the centre axis.
The pitching angle is the angle formed between the centre axis and the roadway surface, strictly speaking the longitudinal direction of the roadway surface. In general, a pitching angle of zero is set if the installation height is between 45 and 65 cm. A greater pitching angle is chosen at a greater installation height.
The azimuth angle is the angle formed between the centre axis and the roadway margin. An expedient azimuth angle is e.g. 22°.
The installation distance is the perpendicular distance between the measuring device and the roadway margin.
The installation height is the perpendicular distance from the intersection point between scanner longitudinal axis and centre axis to the roadway surface. To put it in simpler terms, it is the height at which the laser beam leaves the laser scanner relative to the roadway surface.
In order to be able to compare a method according to the invention with those according to the prior art more simply, the terms defined above are used for this purpose.
DE 199 02 287 A1 describes an automatic method for aligning a scanning plane of a laser scanner mounted on a vehicle with respect to a roadway surface, which method makes use of the fact that a plane can be defined by only three points. This presupposes that the installation height (in that case the height of the transmitter of the laser scanner) relative to the roadway is known. For aligning the scanning plane with respect to the roadway surface, two well-defined bodies are arranged in the scanning range, which reflect a defined value of radiation at a predetermined height, depending on the desired pitching angle (in that case inclination angle), between the scanning plane and the roadway. In the case where an inclination angle of 0° is intended to be set, that is to say that the scanning plane is intended to be aligned parallel to the roadway plane, the body reflects the defined value at the known height of the transmitter of the laser scanner. The position of the transmitter is adjusted until the radiation reflected by the body has the defined value. This method step is then repeated with the other body, as a result of which the rolling angle is set to zero and the position of the scanning plane is defined.
The method is provided for automatically adjusting a laser scanner on an automobile. For a use for aligning a laser scanner installed alongside a roadway, what is disadvantageous about this method, in particular, is that, for aligning the scanning plane, firstly the height of the transmitter relative to the roadway has to be known and secondly, with the two well-defined bodies, special aids are required, the arrangement and measurement of which are difficult in moving traffic. Furthermore, the position of the scanning angle range cannot be set up using this method.
In accordance with a method in the patent specification EP 2 105 761 B1, the scanning plane of a laser scanner can be aligned parallel to a roadway surface without the installation height (in that case the height of the transmitter of the laser scanner) being known. The alignment is effected exclusively by the ascertainment of distance values and presupposes the definition of a rectangular Cartesian spatial coordinate system (in that case roadway coordinate system) and a rectangular Cartesian scanner coordinate system.
The spatial coordinate system can be formed by a roadway longitudinal axis, which runs in the direction of travel parallel to the course of a roadway margin, a roadway transverse axis, which lies with the roadway longitudinal axis in the roadway surface assumed to be a plane, and a normal axis relative to the roadway surface, said normal axis running in the direction of a surface normal.
The installed laser scanner and the thus predefined spatial position of the scanner mirror and also the emission plane of the laser beam reflected by the scanner mirror, which plane constitutes the scanning plane, define the scanner coordinate system. It is determined by a scanning axis, about which the scanner mirror is rotated or pivoted, a centre axis, which bisects the scanning angle range of the laser scanner, and a scanner longitudinal axis, which together with the centre axis spans the scanning plane.
The impingement point of the laser beam on the scanner mirror and thus on the optical axis of a laser that emits the laser beam forms the coordinate origin of the scanner coordinate system.
The coordinate origin of the spatial coordinate system is positioned at a point of intersection of a surface normal—imagined through the coordinate origin of the scanner coordinate system—of the roadway surface through the roadway surface.
The distance between the coordinate origin of the scanner coordinate system and the coordinate origin of the spatial coordinate system defines the installation height.
After the empirical installment of the laser scanner alongside the roadway, the scanning plane is inclined with respect to the roadway plane by a random rolling angle (in that case longitudinal inclination angle) and a random pitching angle (in that case transverse inclination angle) relative to the roadway plane. The centre axis (in that case scanner transverse axis) forms a random azimuth angle with the roadway margin. The random rolling angle and the random pitching angle are small and the random azimuth angle deviates only slightly from 90°.
For aligning the laser scanner, subsequently three method steps, in each case with the aid of measured distance values, involve firstly the random rolling angle being eliminated by the parallel alignment of the scanner longitudinal axis with respect to the roadway longitudinal axis, then the random pitching angle being eliminated by the parallel alignment of the scanning axis with respect to the surface normal of the roadway surface and, finally, the installation height being ascertained as required.
The method for aligning a laser scanner with respect to a roadway in accordance with EP 2 105 761 B1 cited above is concluded once the scanning plane has been aligned parallel to the roadway surface.
This method presupposes that a horizontal alignment of the scanning plane with respect to the roadway is the correct alignment in order to achieve an optimum measurement range.
In practice, however, the installation conditions are highly diverse particularly for mobile measurement devices comprising a laser scanner which are intended to be installed only temporarily, with the result that a horizontal alignment is not always optimal.
Mobile laser scanners, in particular, are intended to be able to be rapidly installed and aligned in an inconspicuous manner and in a manner integrated as much as possible in their environment, often behind a natural concealing screen or a crash barrier as viewed from the roadway. In order to ensure that the laser scanner has an unrestricted view of the roadway despite viewing impediments, it may be of interest to be able to install the laser scanner in a manner such that it is variable in terms of its installation height and thus adaptable to the ambient situation. A different installation height can also arise in an undesired manner from a height difference between the ground surface and the roadway surface.
With increasing installation height, the situation can then arise that reliable measurement is no longer ensured in the case of a horizontal alignment of the scanning plane, such that, in order to arrive at an optimum measurement range, a pitching angle dependent on the installation height has to be set, as a result of which the scanning plane forms a corresponding angle of less than 90° with the roadway surface. The installation distance should also be taken into account for setting a pitching angle of greater than 0°. If said installation distance is always the same within specific tolerances, said installation distance can already be taken into consideration when the dependent pitching angle is generated. Otherwise, predefined pitching angles are stored, which are in each case assigned to an installation height or a tolerance range around the latter and to an installation distance or a tolerance range around the latter.
Although it should not pose a problem for the person skilled in the art to tilt a laser scanner aligned in accordance with EP 2 105 761 B1 cited above by a predefined pitching angle dependent on the installation height, EP 2 105 761 B1 cited above does not give any suggestion in this respect. The installation height is ascertained there possibly only in order to know at what height passing vehicles are measured.
Furthermore, said method does not disclose or suggest aligning the laser scanner at an acute azimuth angle formed between the centre axis and the roadway margin.
At all events, the setting of a pitching angle of not equal to 0° and an acute azimuth angle would require additional method steps, the sequences of which can be neither gathered nor derived from EP 2 105 761 B1 cited above.
DE 10 2011 050 659 B4 cited above discloses a method for aligning a traffic monitoring device, wherein a camera fixedly connected to a measuring sensor of the traffic monitoring device is used to create a plurality of successive images of vehicles travelling through the object region of the camera. An actual horizon line is ascertained in the images on the basis of calculated vanishing points of imaged parallel body edges. The camera is subsequently rotated until said actual horizon line coincides with a desired horizon line which divides the image transversely into two image parts of equal height. The receiver matrix of the camera is then aligned with respect to the desired horizon line, that is to say that the rows of the receiver matrix run parallel to the horizon and thus parallel to the roadway surface. By virtue of the fact that the camera is fixedly connected to a measuring device, a measuring device can also be aligned horizontally by means of this method.
If the measuring device is a laser scanner, by means of this method the rolling angle can be set to zero or eliminated and a predefined pitching angle can be set. A pitching angle dependent on an installation height of the laser scanner cannot be set if said height is not known. Moreover, azimuth angles cannot be set by means of this method.